Economics: Value vs. Processing Cost
A material gets recycled when the value recovered exceeds the cost of collection, sorting, and processing. Aluminum is recycled at near-100% rates because it's highly valuable (saving 95% of primary production energy) and relatively easy to process.
Many plastics (#3–7) have low market value, high processing costs due to variety, and contamination challenges — the economics rarely work without subsidy. The material may be technically recyclable, but the system can't profitably do it at scale.
Material Properties
Physical and chemical properties determine processing difficulty. Aluminum melts easily and can be reformed indefinitely without quality loss. Glass can also be melted and reformed, but different glass types contaminate each other.
Mixed plastics are problematic because different resins have incompatible melting points — you can't melt #1 PET and #2 HDPE together without producing low-quality output. The variety of plastics requires expensive sorting before any recycling can occur.
Collection and Sorting Challenges
Some materials are hard to collect efficiently. Plastic film clogs sorting equipment. Small items fall through sorting screens. Composite materials (products made of multiple materials bonded together) can't be sorted into component streams.
Laminated packaging — juice cartons, potato chip bags, coffee pouches — combines paper, plastic, and metal foil in layers that can't be separated with standard equipment. Specialty processing exists but isn't widely available.
Market Structure
Even materials that can be recycled need buyers. End markets develop when there are enough consistent, quality supply and manufacturers who need the feedstock. Markets for some materials have developed gradually (paper, aluminum, steel); others (multi-resin plastics, composite materials) lack the end-market demand to justify processing infrastructure.
This is why buy-recycled policies, extended producer responsibility laws, and recycled content mandates matter: they create the demand that makes recycling economically viable.